THE THYMUS. 191 



in foetal calves, 2J — 8"' long, wart-like and spherical, in part 

 even shortly pedunculated offsets already existed, which subse- 

 quently increased in number, becoming produced, at first into 

 two and four and afterwards, successively, into still more globu- 

 lar bodies, until ultimately the lobules were completed. In 

 this way the primitive tube would be converted into the central 

 cavity of the thymus and each offset of it, in course of time, 

 into an entire lobule of the organ. In the human subject, as 

 early as in the seventh week, I have seen the thymus lobate 

 at the lower end and single above. In an embryo ten weeks 

 old, the upper extremity was a delicate walled tube, 0*04 — 

 006'" in diameter, filled with polygonal cells; the lower por- 

 tion, 0*\$" thick, presented several rounded outgrowths, in 

 part isolated, in part grouped, to the number of from two to 

 five together. The thicker, inferior portion of the gland, was 

 entirely covered with further developed lobules 0*08 — O'l'" in 

 size, in which, again, more simple gland- granules, each with a 

 structureless membrane and contained cells, were visible. In 

 the twelfth week I found the thymus not much larger, but 

 the horns broader, and, like the rest of the organ, covered with 

 lobules 0-12 — 0-24'" in size. From this, although the first 

 stages have not yet been observed in Man, there can be no 

 doubt that the mode of development is the same as that 

 observed by Simon in other Mammalia. 



The later development of the thymus affords other interest- 

 ing conditions. In the embryo it continues to grow slowly from 

 the third month; in the sixth it extends as far as to the 

 thyroid gland ; and from and after the seventh month begins 

 to contain a whitish secretion. After birth it is not stationary, 

 as was formerly believed, but usually continues to grow until 

 the second year, and, indeed, at first very considerably. After 

 that period its growth ceases, though it still usually remains 

 for some time longer unchanged, until ultimately it becomes 

 atrophied, and finally disappears. The period at which these 

 changes take place varies very much. Simon places the 

 commencement of the atrophy between the eighth to the 

 twelfth years, a statement which, from my own observations, 

 and in accordance with those of Ecker, I cannot consider as 

 universally correct ; because, up to the twentieth year, the 

 thymus is frequently met with in a good state of nutrition, 



